1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases from an internal combustion engine of an automobile and, more particularly, an exhaust gas purifying catalyst for removing effectively high concentration hydrocarbons exhausted in engine start-up.
2. Description of the Related Art
In the prior art, for the purpose of purifying the exhaust gases from the internal combustion engine of the automobile, etc., the three-way catalyst that can perform oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx) simultaneously has been widely employed.
A large amount of HC, which is called xe2x80x9ccold HCxe2x80x9d, are exhausted at the low temperature in the engine start-up. However, since the exhaust gas temperature must be kept at more than 300xc2x0 C. to make the above three-way catalyst active, the three-way catalyst is inactive at the low temperature immediately after the engine start-up. Therefore, if only the three-way catalyst in the prior art is employed, the cold HC is not purified but exhausted as it is.
In recent years, for the purpose of purifying this xe2x80x9ccold HCxe2x80x9d, the HC adsorbing catalyst employing the zeolite has been developed.
The HC adsorbing catalyst is such a catalyst that adsorbs and holds temporarily the cold HC in the engine start-up during when the three-way catalyst is not activated, and then desorbs HC gradually to purify HC by using the three-way catalyst when the three-way catalyst is activated after the temperature of the exhaust gas is increased.
Such HC absorbing catalysts are disclosed in Japanese Laid-Open Patent publications Hei 6-74019 published in 1994, Hei 7-144119 published in 1995, Hei 6-142457 published in 1994, Hei 5-59942 published in 1993, Hei 7-102957 published in 1995, and Hei 11-104462 published in 1999.
In the exhaust gases, there are various HCs having different molecular weights. Since the HC adsorbing catalyst using the zeolite has a correlation between an HC distribution in the exhaust gases and a pore diameter in the zeolite, the zeolite having the optimum pore diameter must be employed.
In the prior art, the pore diameter distribution is adjusted by blending the MFI-type zeolite as the main element with the zeolite (e.g., USY type) having another pore diameter. However, since the distortion of the pore diameter and the adsorption/ desorption characteristic are different after long time use according to the zeolite type, adsorption of the HC in the exhaust gases becomes insufficient.
Also, in the prior art, a three-way catalyst contains noble metals such as platinum (Pt), palladium (Pd), rhodium (Rh), etc. mixed in the same layer and another three-way catalyst contains a Rh layer and a Pd layer coated separately, etc. have been proposed for the noble metals in the three-way catalyst. Japanese Laid-Open Patent Publication Hei 2-56247 published in 1990 discloses an exhaust gas purifying catalyst in which a second layer containing noble metals such as Pt, Pd, Rh, etc. as the main component and having the three-way catalytic function is provided on a first layer containing the zeolite as the main component.
In the exhaust gas purifying catalyst having the structure in which the zeolite layer as the HC adsorbent and the three-way catalyst layer are laminated on the monolithic support in the prior art, following problems are pointed out.
In the low temperature zone of the exhaust gases immediately after the engine start-up, such three-way catalyst layer prevents the cold HC diffusing into the zeolite layer and thus makes worse the adsorption efficiency of the cold HC.
In addition, since a thickness of the zeolite layer coated in the cells of the monolithic support is not uniform, HC adsorbed in the zeolite is quickly desorbed at the thin portion of the zeolite layer with the increase of the exhaust gas temperature and the increase of the exhaust gas flow rate. If desorption of HC is too quick, HC cannot be effectively purified in the overlying three-way catalyst layer.
Further, in order to make the three-way catalyst effectively fulfill its purifying function, the air-fuel ratio (A/F) must be set in the vicinity of a stoichometric air-fuel ratio (A/F=14.6) at which the oxidation of HC and CO and the reduction of NOx are balanced. However, in the exhaust gas purifying catalyst in which the three-way catalyst layer is laminated on the zeolite layer, since HC which has been adsorbed into the zeolite layer in the low temperature zone of the exhaust gases immediately after the start of the internal combustion engine is abruptly desorbed with the increase of the exhaust gas temperature, the exhaust gas becomes fuel-rich. Therefore, the three-way catalyst does not fulfill sufficiently the purification function, the purification of HC, Co, NOx is not performed with good balance.
In the prior art, no particular study of a structure of the zeolite layer coated in the cell of the monolithic support has been made. However, according to the study made by the inventor of the present invention, it is found that, unless the optimization of the zeolite layer structure can be attained in the HC adsorbing catalyst, the HC adsortion/desorption/purification cycle cannot be effectively carried out.
Moreover, according to the study of the inventors of the present invention, when the zeolite layer is coated in the monolithic support having polygonal cells, the inner wall of the coating layer is formed substantially like an inscribed circle of the polygonal cell. In other words, a thickness of the coating layer is thick at the cell corner portion while a thickness of the coating layer is thin at the cell side portion. Therefore, there is such a tendency that the adsorption/desorption characteristic of the zeolite in the cell side portion is considerably degraded rather than that in the cell corner portion.
In order to achieve the uniformization of the thickness of the zeolite layer, it may be considered that the monolithic supports having the cells whose inner wall sectional shape is a circle is employed. In this case, a thickness of the wall portion corresponding to the cell corner portion is increased and thus a volume of the support itself is increased. As a result, since a heat capacity of the support is increased and then the temperature increase of the three-way catalyst is delayed, the purification efficiency of HC desorbed from the zeolite layer is considerably degraded.
Besides, in the three-way catalyst for purifying HC desorbed from the HC adsorbent, a large quantity of noble metal are employed to maintain the high purification performance from the beginning to the endurance time, or an air is introduced from the outside to achieve the quick activation. Therefore, the catalyst that enables the high performance by the reduced amount of the noble metal is desired. However, if the amount of the noble metal is reduced, the durability of the catalyst becomes insufficient and thus the catalytic activity and the purification performance are deteriorated in the low temperature zone after the endurance time.
In view of the above subjects in the prior art, it is an object of the present invention to provide an exhaust gas purifying catalyst capable of achieving purification of HC, CO, and NOx with good balance and improving the purification performance of cold HC by controlling the diffusion (speed) of the exhaust gases that pass through cells in a support and diffuse into coating layers.
In order to achieve the above object, according to an aspect of an exhaust gas purifying catalyst of this present invention, such exhaust gas purifying catalyst comprises a monolithic support having a plurality of cells each of which has a polygonal sectional shape, a first layer formed on a monolithic support to contain heat resisting inorganic material, a second layer formed on the first layer to contain hydrocarbon adsorbent, and a third layer formed on the second layer to contain a metal-based catalyst. Where the second layer has a ratio Lmax/Lmin of the thickest portion (Lmax) at corner portions of the cells to the thinnest portion (Lmin) at flat portions of the cells in a range of 1 to 10, and the second layer has a thickness of 10 to 500 xcexcm.
As the heat resisting inorganic material, ceramic materials such as alumina, titania, zirconia, etc. may be listed.
According to the above aspect of this present invention, the second layer containing hydrocarbon adsorbent as the main component (called the hydrocarbon adsorbent layer hereinafter) is not directly formed on the monolithic support but formed on the first layer containing the heat resisting inorganic material as the main component. The first layer thickly covers the cell corner portions of the support, and brings inner walls of the cells close to the circular shape. Therefore, the thickness of the hydrocarbon adsorbent layer can be adjusted into the appropriate range.
Accordingly, HC adsorption/desorption/purification cycles are carried out effectively, and such HC adsorption/ desorption/purification can be performed uniformly over the entire second layer without deviation.
Also, a catalytic component layer containing Pd may be employed as the third layer and then an insoluble salt of an alkaline earth metal may be contained in this layer. Sintering of the noble metal can be suppressed and thus the low temperature activity and the purification performance can be improved.
In addition, if a layer containing rhodium is laminated as a fourth layer on the third layer, the cold HC adsorbing/desorbing ability characteristic and the desorbed HC purification performance can be effectively exhibited.
Furthermore, platinum (Pt) may be mixed in the third layer and the fourth layer. Therefore, the poisoning resistance of the third layer as the catalytic component layer (three-way catalyst layer) can be improved.